4 CHARLES F. W. McCLUBE 



may enable us to determine the exact time at which the macro- 

 phages and other tissues of the embryo first begin to react toward 

 the dye, as well as the causes which underlie the initiation of such 

 reactions. We may also be enabled to study the progressive 

 reactions of the tissues toward these dyes during the different 

 stages of larval development. It is possibly not quite clear what 

 the exact source of origin of all the different types of white blood- 

 corpuscles may be that are found in the embryonic body. As the 

 endothelium of the lymphatic system, and, to a considerable ex- 

 tent, also that of the venous system has been found to be vitally 

 stained by these dyes in young toad larvae, it may now be possible 

 to distinguish more clearly, than has hitherto been the case, be- 

 tween the white blood-cells of strictly endothelial origin and those 

 which arise directly from mesenchyme. The portal of entry and 

 transportation of the dye in larval and adult amphibia, as well as 

 the reactions of the tissues toward colloidal acid dyes during the 

 period of hibernation, are also among the questions which have 

 suggested themselves as worthy of further inquiry. 



The present investigation was begun chiefly with the intention 

 of examining the claim made by Wislocki that his method 

 promised to throw additional light on the development of the 

 lymphatics. Whatever claims he has thus far made in this direc- 

 tion seem to be in accord, both for the development of the lym- 

 phatics and blood-vessels, with the old and disproved angioblast 

 theory of His. In fact, he specifically states that the theory of the 

 venous origin of the lymphatics which he claims to confirm, is in 

 direct accord with the earlier work upon the development of 

 blood-vessels by His. 



My original plan has been greatly modified and extended, as so 

 many other important questions presented themselves during 

 the course of the investigation. In addition, therefore, to the 

 question of lymphatic development, the above-mentioned in- 

 quiries, with one exception, will largely form the subject of this 

 paper. This exception is a more detailed account of the develop- 

 ment of the white blood-corpuscles,, which may best be con- 

 sidered in a separate publication with illustrations. 



